The purpose of the enclosure seemed to be to maintain an area of clear air in the midst of the swirling purple vapors that pressed in against it from the top and from every side. In shape it was a great oblong cell, some fifty feet high, two hundred yards long, and about one hundred yards wide. The three captives stood near the center. Fencing in the enclosure at twenty-yard intervals and reaching upward to the ceiling were slender posts of some lusterless black metal. Between these posts streamed unbroken, nearly transparent sheets of some unknown force, the only visible sign of which was the presence of countless millions of tiny shimmering golden flecks which danced like dust motes in a ray of sunlight. It was obviously this thin sheet of unknown force that was keeping the purple mists at bay, for fan-like antennae at the top of each post spread a similar shimmering sheet that formed a ceiling for the clear-aired area. * * * * * The three Earthlings were facing one of the side walls of the big enclosures. The purple mists outside made it hard to see clearly for any distance, but Blake had an impression that the surrounding terrain was featured by the same barren, nearly desert bleakness that characterized the interior of the enclosure, where scattered clumps of dead, spiky black branches of shrub-like vegetation were the only sign of plant life. Just within the distant end wall at their right there was a low platform surmounted by a wide arch some ten feet in height, both constructed of silver-colored metal. There was nothing between them and the end wall to their left, but they could see that the ground sloped sharply upward from the barrier-sheet, and on the crest of the ridge a gigantic cone-shaped structure of solid black could be seen dimly through the intervening mists. The cone-building seemed to be the source of the power that kept the enclosure intact. Slender cables of black metal ran down the slope from it into the clear-aired space, spreading out over the dusty gray-blue ground to the base of each of the tall posts, with a heavier copper-colored cable running on the silver arch. From within the windowless interior of the cone there was audible a low hum as of tremendous power being generated there. "Gee, what a rummy joint this place is!" There was frank awe in the gangster's voice as he at last broke the silence. "That guy with the green net sure took us for one sweet ride. Mebbe we're on the Moon now, or on Mars, huh?"